An active matrix organic light-emitting diode (AMOLED) display is considered as a mainstream display technology of the next generation because of its advantages of a rapid response, high brightness, high contrast, low power consumption and easy implementation of flexibility and transparency, etc. Recently, a large number of studies have been carried out to promote the large scale production of the AMOLED display.
In designing an AMOLED panel, a major problem that needs to be solved is the non-uniformity of brightness among pixels.
Firstly, a thin film transistor (TFT) is employed in the AMOLED display to construct a pixel circuit to provide an OLED device with a corresponding current. The thin film transistor is often in a form of a low temperature polycrystalline silicon thin film transistor (LTPS TFT) or an oxide thin film transistor (Oxide TFT). As compared to a common amorphous silicon thin film transistor (amorphous-Si TFT), the LTPS TFT and the Oxide TFT have a higher mobility and more stable characteristics, and are more suitable for being applied in the AMOLED display. However, due to the limitations of a crystallization process, an LTPS TFT produced on a large area glass substrate often exhibits non-uniformity in terms of electrical parameters such as the threshold voltage, mobility, and such non-uniformity will be converted to a current difference and brightness difference of the OLED device, and thus perceived by human eyes (namely, a mura phenomenon). The uniformity of the process of the Oxide TFT is better. But similarly to the a-Si TFT, after being under a pressure and high temperature for a long time, the Oxide TFT will experience a drift of the threshold voltage. In displaying different images, the amounts of threshold voltage drift of the TFTs of individual parts of the panel are different, which will result in a display brightness difference. Since such a difference is related with an image displayed previously, it often leads to rendition of artifacts.
Secondly, when the OLED device is in evaporation, non-uniformity in thickness of the film will also cause the non-uniformity of the electrical performance. For an a-Si TFT or Oxide TFT process in which an N-type thin film transistor (N-Type TFT) is employed to construct a pixel cell, a storage capacitor is connected between a gate of the drive TFT and the OLED anode. Thus, when a data voltage is transmitted to the gate, if the voltages of the OLED anodes of individual pixels are different, the gate-source voltages (Vgs) actually loaded onto the TFTs will be different, and thereby the drive current will be different, causing a difference in the brightness of the pixels of the OLED display.
Studies show that, the main reasons that cause a difference in display brightness of the AMOLED panel lie in that the threshold voltages of OLED drive transistors in different pixel cells are different due to the drift, and that the working voltages of OLED devices in different pixel cells are different due to non-uniform film thicknesses of the OLED devices in evaporation.
To solve the problem of a display brightness difference in the AMOLED panel, in general, the threshold voltage of the drive transistor and the working voltage of the OLED device will be compensated by way of an internal compensation or external compensation. However, before the compensation, it is needed to detect the threshold voltage of the drive transistor and the working voltage of the OLED device first. How to detect the threshold voltage of the drive transistor and the working voltage of the OLED device in an AMOLED panel has become an urgent problem to be solved.